110 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



the Knobstone escarpment at a point where it has a rehef of over 200 feet 

 above the shale basin. It then crosses a shallow basin in the St. Lonis 

 limestone, after which it passes throug-h the prominent Kaskaskia and 

 Cong-lomerate Coal Measure formations and reaches the low bashi of the 

 friable Coal Measures. Finally it traverses the elevated rim of Lower Car- 

 boniferous sandstone and limestone and enters the broad valley of the 

 Lower Mississippi. 



This disregard of topographic features is paralleled by some of the 

 tributaries of the Lower Ohio. Green River takes a somewhat direct west- 

 ward course across escarpments and basins in the formations between the 

 Devonian and Coal Measures in a district south of the Ohio, while the East 

 White River takes a similar course in a district north of the Ohio. Salt 

 River passes westward across the low basin of Devonian shale into the 

 Knobstone escarpment and joins the Ohio instead of turning northward 

 along the axis of the low basin. The Kentucky River seems to have 

 suffered more deflection by topographic and structural features than the 

 tributaries just mentioned. In crossing the crown of the Cincinnati arch 

 it makes a marked detour to the southwest, but turns back to the north 

 along the east side of the low Niagara escarpment, as if guided by that 

 escarpment. 



While the streams of this drainage system usually pass somewhat 

 directly across the basins and escarpments, their courses are generallj^ 

 marked by a pronounced breaking down both of the escarpments and of 

 the basins in the vicinity of the stream valleys. This is especially true of 

 the major stream, the Ohio. In the passage across the Lockport (Niagara) 

 limestone near Madison, Ind., the immediate bluffs rise 350 to 400 feet 

 above the stream, or 750 to 800 feet above tide, but the uplands 2 or 3 

 miles back reach fully 500 feet above the stream, and become still higher 

 farther back. In the Devonian shales the Ohio bluffs are only 75 to 125 

 feet above the river, or 450 to 500 feet above tide, but back 20 miles from 

 the Ohio, at the divide between this river and the East White River, the 

 axis of the shale basin rises to an altitude of over 600 feet aitove tide, and a 

 similar rise is found south of the Ohio along the axis of the basin. The 

 Knobstone formation is much more broken or degraded in the vicinity of 

 the Ohio than a few miles north or south of the river, as well as of lower 

 altitude. Its highest knobs on the border of the Ohio scarcely reach 800 



